One major problem facing optical networks based on wavelength Division Multiplexed (WDM) technologies with fiber amplifiers is power transients. Power transients can arise from a number of sources and generally refer to a change in power of a subset of wavelength channels propagating through a system. Power transients can occur due to planned or unplanned transient events in an optical network. These events may include changes in a number of wavelength channels and transient/permanent deformation of a fiber (e.g., connector pull, fiber bend, fiber cut, amplifier failure, signal laser failure, etc.).
A number of wavelength-multiplexed channels traversing an erbium-doped fiber amplifier (EDFA) in a multi-wavelength network may change as a result of network configuration, network growth, or network faults. EDFAs are generally operated near gain saturation where a total output power at saturation is very nearly constant, independent of the number of channels. All channels present are amplified simultaneously by an amplifier and must share the available saturated gain. An output power of each channel will, therefore, depend on the number of channels present. Channel count changes will induce transients in the gain at other wavelengths, via transient cross-saturation in the amplifier. System performance may be degraded by fiber nonlinearity when channel powers are too high. Loss of channels can give rise to surviving channel errors (erg., increased bit error rates) since the power of the surviving channels may surpass thresholds at which system nonlinearities become significant. Addition of channels can cause similar errors by depressing the power of the surviving channels below a receiver sensitivity. Performance degradation may occur due to small receiver signal-to-noise ratio (SNR) when channel powers are too low.
Signal power fluctuations may also occur due to span loss variations or random polarization changes. Unplanned transient events such as those due to failure or accidental handling of optical fiber (e.g., connector pull, fiber bend, fiber cut, amplifier failure, etc.) may result in complete failure loss of an optical signal and/or a reduction of power in a fiber.
Essentially, power excursions due to planned or unplanned transient events may cause power of other channels propagating through a given EDFA to change, which in turn may cause errors or even damage to a receiver. Thus, EDFAs require effective gain control mechanisms in order to limit effects of power excursions in a network. Also, since these power excursions increase with the number of EDFAs (e.g., signal degradations may accumulate along an amplifier chain) and seriously degrade transmission performance, gain control of EDFAs is a key requirement for achieving reliable WDM networks. Cross-saturation in a network with a chain of EDFAs may induce power transients to surviving channels at a speed which is proportional to a number of amplifiers in the chain. While typical time scales for gain changes in a single amplifier are tens of microseconds, a time constant for a chain of N amplifiers is 1/N times shorter than that of a single amplifier. Thus long chains of amplifiers will require faster control to limit undesirable power excursions, presenting a greater stabilization challenge.
Several automatic (or dynamic) gain control schemes have been proposed to suppress power transients. These include gain clamping by an all-optical feedback loop, fast pump control, and fast link control by insertion of a control channel.
In an all-optical gain clamping method, a portion of an amplifier (e.g., EDFA) output is tapped off, filtered by a band pass filter, and fed back into the amplifier. The gain of a loop is controlled by using an attenuator in the loop. This feedback loop causes the amplifier to lase at a wavelength passed by the filter in the loop. This has the effect of clamping an amplifier gain seen by other wavelengths to a fixed value, irrespective of an input signal power.
Fast pump control has been shown to limit power excursions of surviving channels and protect surviving channels against fast power transients. In a fast pump control method, a simple automatic gain control (AGC) circuit monitors an input power into an amplifier and adjusts a pump power to vary a gain if the input power changes. The response time of this method is limited ultimately by the lifetime of electrons from the third energy level to the second energy level in Erbium which is around 1 As.
In a fast link control method, an additional wavelength is introduced on a link to act as a compensating wavelength. The compensating wavelength is introduced at a beginning of the link and tapped off at an end of the link. The power on the compensating wavelength is increased to compensate for any decrease in power seen at the input to the link.
Proportional integral (PI) controllers are conventionally used with feedback control for controlling power gain of an optical amplifier and to eliminate disturbances. PI controllers require a linear mathematical model of a process, which is typically developed using a priori information of the process. However, PI controllers are not suitable for processes with time-varying parameters and nonlinear processes.
Some PI controllers have been enhanced with a feed-forward control loop. With this enhancement, it is possible, in some situations, to measure disturbances before they have influence on the process. In this case, the effects of the disturbances can be eliminated before control errors are created. Feed-forward control, however, requires a precise mathematical model of a process and is typically much more sensitive to modeling errors than a simple feedback control scheme.
An optical amplifier may be used to compensate for intensity loss in fiber and network elements. Optical amplifiers may operate in saturated mode and cause a cross-saturation effect on surviving channels when a number of supported channels is changed due to link reconfiguration or failure thereby creating signal power transient in surviving channels. To maintain quality of an optical signal, power gain of an optical amplifier must be properly controlled to eliminate channel errors caused by surviving channel signal power transients.
In view of the foregoing, it would be desirable to provide a technique for adaptively controlling gain in an optical amplifier which overcomes the above-described inadequacies and shortcomings. More particularly, it would be desirable to provide a technique for adaptively controlling gain in an optical amplifier in an efficient and cost effective manner.